GMRT was one of the pioneering instruments for CD/EoR science, placing the first upper limit on the 21-cm power spectrum at redshift \(z\sim 8.6\)  [1]}, using \(\sim 40\)  hours of observation with the legacy GMRT. The data was calibrated using a pulsar B0823+26 as a calibrator. Following calibration, RFI removal, and foreground subtraction, data from multiple observing nights were cross-correlated to set the upper limit. A revised upper limit, after accounting for signal loss due to foreground subtraction, was placed by [2]}. This was still the most stringent upper limit on EoR 21-cm signal at that time. In addition to these observations and the corresponding upper limits on the Hi 21-cm power spectrum at EoR redshift, quite a significant amount of work have been done using GMRT at frequencies corresponding to the post-EoR epoch. The challenge of extremely bright foregrounds is same for both EoR and post-EoR Hi signal detection. Hence, the knowledge gathered from the post-EoR observation can be used to EoR observation and vice-versa. [3]} used GMRT observations at 618 MHz to put the first upper limit on [\( \bar{x}_{\mathrm {HI}} b_{\mathrm {H{\sc I}}}\) ] \(\le \)  2.9 ([\(\Omega _{\mathrm {H{\sc I}}} b_{\mathrm {H{\sc I}}}\) ] \(\le \)  0.11) at \(z \sim 1.32\) , where \( \bar{x}_{\mathrm {HI}}\)  is the mean neutral fraction. The uGMRT data centred at 400 MHz was used by [4]} to place the first upper limits on the post-EoR 21-cm PS in the range \(2\lesssim z \lesssim 4\) . They also constrained [\(\Omega _{\mathrm {H{\sc I}}} b_{\mathrm {H{\sc I}}}\) ] with upper limits 0.09,0.11,0.12,0.24 at \(z=1.96,2.19,2.62,3.58\) , respectively.
